Multiple access communication systems are well understood in the art. Multiple access communication systems are designed to provide access to limited communication resources by a plurality of communication units for the purpose of transmitting communication messages, referred to as packets. The access methodology, referred to as a multiple access protocol, is chosen such that some appropriate set of performance constraints are met. Typical performance constraints include efficiency of communication resource use, communication message delay, and other similar factors. Multiple access protocols can generally be regarded as belonging to one of two general types, contention and non-contention.
Non-contention protocols are designed such that a communication unit desiring to send a packet is permitted exclusive use of a communication resource. One example of this type of protocol is time-division multiple access (TDMA) where the communication resource is divided into a plurality of time frames that are further subdivided into a plurality of time slots and each communication unit is assigned exclusive use of one or more time slots in each time frame. This type of protocol is inefficient for communication units that source substantially infrequent messages since the assigned time slot remains substantially unused by anyone in between messages. The practical number of communication units that can be accommodated by such a protocol is also limited by the delay incurred while waiting for one's assigned slot. This wait usually increases proportionally to the number of communication units that have assigned slots.
Contention protocols are characterized by communication units that actively compete with each other to gain access to the communication resource. The slotted ALOHA protocol is an example of this type of protocol. In slotted ALOHA, a communication resource is divided into a plurality of time slots. A communication unit desiring to send a packet may transmit in the first subsequent time slot, taking care not to transmit outside of the boundaries of that time slot. If no other communication unit also transmitted in that same time slot, the packet transmission is considered successful. (Note that other factors, such as communication channel noise, may ultimately result in failure of the message, but that these other factors are not related to the access protocol.) If one or more other communication units, however, did transmit a packet in the same time slot, generally all transmissions would fail due to collision. Thus contention protocols generally work well for lightly loaded systems, but performance suffers as load increases because the likelihood of collisions also increases. Further, communication messages longer than the time slot duration must be sent in a plurality of time slots and are subject to collision in each time slot used.
Reservation protocols, a sub-class of contention protocols, are also known. Reservation protocols attempt to combine certain aspects of contention and non-contention protocols to provide improved performance for a wider variety of communication system conditions. A typical reservation protocol divides a communication resource into a series of fixed-size time frames further divided into a series of time slots. The time slots are comprised of two types, a reservation time slot and a data time slot, with equal numbers of each in each time frame. The reservation time slots are generally smaller than the data time slots and are grouped together at the beginning of each time frame. A communication unit desiring access to the communication resource transmits randomly in one of the reservation time slots for the purpose of reserving an associated data time slot. If the unit successfully avoids contention and is therefore the only unit to transmit a reservation request in a given reservation time slot, it is permitted exclusive access of the associated data time slot occurring later in the time frame.
Although reservation protocols improve the effectiveness by which a communication resource may be utilized by a plurality of competing communication units, particularly when there are a wide range of communication requirements, some drawbacks exist with these schemes. A communication unit wishing access to the channel must first wait for the reservation time slots. If no messages are currently being sent, this represents a delay which would not have occurred if the unit had been allowed to transmit immediately, in random access fashion. Further delay is encountered between the time the unit successfully accesses the reservation time slot, via random access, and the time it receives confirmation of its reservation.
One particular reservation protocol, the Reservation-ALOHA (R-ALOHA) protocol, should be mentioned. Like TDMA, the communication resource is divided into time frames that are further divided into time slots. When the communication resource is unused, however, the protocol operates similarly to slotted ALOHA. When a communication unit desires to send a packet, it transmits in one of the unused time slots, referred to as a random access slot. If the transmission is successful, i.e., it does not collide with another transmission, the communication unit is permitted exclusive use of that same time slot in subsequent time frames, referred to as reserved access slots until the packet is completely transmitted. Thus the initial ALOHA transmission results in a subsequent reservation of a communication resource. (Note that some method of feedback to the communication units regarding the success or failure of initial ALOHA transmissions is necessary in order for this protocol to be effective.)
R-ALOHA is quite efficient for communication systems accommodating a wide variety of packet frequencies and sizes. However some limitations can be noted. The ultimate efficiency of the protocol is governed by the size of the random access portion of a packet relative to the complete packet, since it is only this portion that is subject to contention failure. In R-ALOHA, this size is equivalent to a time slot. There are many competing factors that contribute to the determination of time slot duration in the design of a communication system. The final embodiment may not result in optimum access protocol performance. For example, longer time slots increase transmission efficiency because requisite overhead requirements are reduced, but a longer time slot decreases the effectiveness of R-ALOHA. Also due to the contention for unused time slots in R-ALOHA, several unused slots may need to pass before a successful random access is accomplished. These unused slots represent wasted communication capacity.
Accordingly, a need exists for a communication method that provides increased utilization of a communication resource by a plurality of communication units with widely varying communication requirements. This need can be substantially met by a communication method that provides an opportunity for communication units to choose from a set of available multiple access methods.